The unexpected endorsement gives Icahn a carrot to dangle before Yahoo shareholders as he wages an acrimonious campaign to replace Yahoo's nine directors at the company's annual meeting Aug. 1.

It marks the first time that Microsoft has publicly sided with Icahn since the billionaire investor launched his attempted coup nearly eight weeks ago.

The two sides decided they could work together after Icahn held "frequent" discussions with Microsoft Chief Executive Steve Ballmer and some of his top lieutenants during the past week, according to a letter that Icahn sent Monday to Yahoo shareholders.

Industry analysts said Icahn now has more credibility with Yahoo shareholders, because he has been arguing that a purge of Yahoo's board is the only way to salvage a deal with Microsoft.

"This breathes new life into Icahn's proposal," said Stanford Group analyst Clayton Moran. "It really pushes the power to Icahn and his board (nominees)."

The prospect that a changing of the guard at Yahoo might pave the way to a friendly deal with Microsoft lifted Yahoo shares $2.56, or 12 percent, to finish Wednesday at $23.91.

"If Microsoft and Mr. Ballmer really want to purchase Yahoo, we again invite them to make a proposal immediately," Yahoo said.

But Icahn said Microsoft doesn't want to risk making a bid under Yahoo's current regime, because the software maker fears Yahoo's management would make more poor decisions during an antitrust review that would take at least nine months.

"If the current board and management team of Yahoo mismanage the company (and their recent track record is far from reassuring), Microsoft would be putting its money at risk, and a great deal could be lost," Icahn wrote in his letter to shareholders.

Microsoft's willingness to work with Icahn undermines one of Yahoo's chief arguments for re-electing its board.

Yahoo has maintained that it would be foolhardy to back Icahn's slate of alternate nominees because Icahn had no concrete ideas besides selling the company to Microsoft  something that Yahoo has been depicting as a pipe dream since Microsoft withdrew a $47.5 billion offer in early May.

Microsoft reinforced that perception by refusing to revive its bid last month, even after Yahoo's board signaled its willingness to accept the earlier offer.

With Microsoft in Icahn's corner, "the dynamic has changed," Sanford C. Bernstein & Co. analyst Jeffrey Lindsay said. "There is now a rationale for voting for Icahn's board, because there now seems to be a real possibility for a deal again."

Monday's turn of events amplifies the pressure on Yahoo co-founder and CEO Jerry Yang, whose handling of the earlier negotiations with Microsoft infuriated many shareholders.

Yahoo's stock price had plunged by more than 30 percent to fall below $20 during Yang's first six months as CEO. Then, in January, Microsoft raised hopes for a quick windfall with its unsolicited takeover bid, only to be repeatedly rebuffed.

If he seizes control of the board, Icahn has promised to fire the 39-year-old Yang as CEO and replace him with a more seasoned leader.

Yang has been meeting with Yahoo's major stockholders during the past week, hoping to persuade them to give him a chance to prove the Sunnyvale-based company is worth more than the $33 per share that Microsoft previously offered.

Ballmer withdrew that bid after Yang sought $37 per share  a height the stock hasn't reached in 2 1/2 years.